This invention relates to a highly sensitive heat-sensitive recording material excellent in thermal response.
A heat-sensitive recording material generally comprises a support and formed thereon a heat-sensitive recording layer composed mainly of an electron donative, colorless or light-colored dye precursor and an electron-accepting color developer. Upon heating with a thermal head, a thermal pen, a laser light or the like, the dye precursor instantaneously reacts with the color developer to give recording images. Such a system is disclosed, for example, in Japanese Patent Examined Publication Nos. 43-4160 and 45-14039. Such heat-sensitive recording materials have been used over a wide range of fields such as measuring recorders, facsimile machines, printers, terminals of computers, labels, automatic ticket vending machines, etc., because the recording can easily be made using a relatively simple device, the maintenance is easy, noises are not produced, and the like. Particularly in facsimile machines, a great demand for heat-sensitive type continues to expand and at the same time, the speed of the machine is getting faster and faster because of necessity for reducing the transmission cost. In response to such a high speed facsimile, the demand for higher sensitive heat-sensitive recording materials is increasing.
In order to transmit and receive a standard original of A-4 size in several to 20 seconds in high speed facsimile machines, it is necessary to repeatedly apply a current to the thermal head in such a very short period of time as several milliseconds or less, and the heat energy generated thereby is transmitted to heat-sensitive recording sheet in which the reaction for forming images is carried out.
In order to carry out the reaction for forming images by the heat energy transmitted in such a short period of time, it is required that the heat-sensitive recording material is excellent in thermal response. To enhance the thermal response or reactivity, compatibility of a color developer with a dye precursor should be improved. For this purpose, sensitizers are used depending on necessity. The sensitizers have an action to accelerate the color-forming reaction by dissolving or enveloping therein dye precursors and color developers present around them when the sensitizers themselves melt with the transferred heat energy. For increasing the sensitivity of the heat-sensitive recording material, it is one method to improve the thermal response of the sensitizers or compatibility of the sensitizers with the dye precursors or color developers.
As such a technique, there are disclosed methods for adding waxes in Japanese Patent Unexamined Publication No. 48-19231; for adding nitrogen-containing compounds, carboxylic acid esters, etc., in Japanese Patent Unexamined Publication Nos. 49-34842, 50-149353, 52-106746, and 53-5636; for adding naphthol derivatives in Japanese Patent Unexamined Publication Nos. 57-64593 and 58-87094; for adding naphthoic acid derivatives in Japanese Patent Unexamined Publication Nos. 57-64592, 57-185187, 57-191089 and 58-110289; for adding p-benzyl-biphenyl in Japanese Patent Unexamined Publication No. 60-122193; for adding 4-allyloxybiphenyl in Japanese Patent Unexamined Publication No. 61-272189; for adding a diphenyloxyethane in Japanese Patent Unexamined Publication No. 60-56588; and for adding a sulfide in Japanese Patent Unexamined Publication No. 61-242884.
However, the heat-sensitive recording materials produced by these methods are still insufficient in developed color density and color development sensitivity.